Abstract

Torsional loads on drill strings can reduce the axial compressive load limit for buckling the drill string. The larger the torsional load, the lower the buckling load. This paper presents a new analysis of drill-string buckling under axial compressive and torsional loads. New equations for buckling loads and lengths are derived for drill strings in vertical, inclined, horizontal, and curved wellbores.

The new analysis includes a model of the work done by the torsional load in the buckling process and provides a better understanding of drill-string buckling under the combined compressive and torsional loads. The results also agree with a previous study for horizontal wellbores.

With the new equations, drill-string buckling, drag, and torque can be predicted more easily and accurately for drilling horizontal wells. An example calculation is provided to illustrate the use of the equations for predicting drill-string buckling while rotary-drilling horizontal wells.

Introduction

Tubular buckling remains an important and interesting issue in the oil industry. Most studies assume no torsional loads are present. For drill strings, torsional loads always exist, raising the question of how much the buckling load is affected by the torsional load. A previous study by Miska and Cunha stated that torsional loads could reduce buckling loads by 10%. This paper presents an additional analysis of drill-string buckling under axial and torsional loads in vertical, inclined, horizontal, and curved wellbores. Drill-String Buckling in Vertical Wellbores Consider a drill string under an axial compressive load, F, and a torsional load, Tn (Fig. 1). From the straight condition to the helically buckled condition, the increase in bending energy is calculated according to the curvature of helical buckling deflection, C4:

(1)

The work done by the axial load is calculated as the product of the average axial load, F in the helical buckling process and the relative axial movement of the drill string due to helical buckling:

(2)

The work done by pipe weight is calculated using one-half of the relative axial movement of the drill-string due to helical buckling:

(3)

The work done by the torsional load is calculated using the bi-normal component of the torsional load, Tn2, along the drill string (Fig. 1):

(4)

The tangential component of torsional load along the drill string does not affect bending, and there is no normal component of the torsional load along helically buckled drill strings. It should be noted that helical buckling should be in the same direction as the torsional load.

According to energy conservation, we have:

(5)

which results in the following (with p =):

P. 703^

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